The average kinetic energy for the O2 molecule (rigid rotator) is

According to equipartition of energy, average energy absorbed by a molecule for each degree of freedom = 1/2 KBT.O2 molecule has 2 degrees of freedom for rotational motion and 3 degrees of freedom for translational motion.So f = 3 + 2 = 5.Therefore, total average kinetic energy possessed by O2 molecule = fKBT/2 = 5 KBT/2.